1. Field of the Invention
The present invention relates to a semiconductor module directed to an IGBT module of a power conversion apparatus for application to a power supply in rolling stocks and the like. The invention relates particularly to an insulation structure of a control terminal block formed on a molded resin casing of the module.
2. Description of the Related Art
Recently, the power conversion apparatuses described above employ a three-level PWM inverter in many cases. Patent Document 1 discloses a three-level IGBT module for application to such an inverter device; the module is a four-in-one type IGBT module containing four sets of semiconductor switching elements each comprising an anti-parallel-connected IGBT and FWD, and integrated in one package in a molded resin casing. The patent application of Patent Document 1 was done by the same applicant as that of the present patent application to the Japanese Patent Office.
FIGS. 4 through 7 show an example of conventional construction of a single phase inverter unit (that is a general purpose inverter unit and not dedicated to rolling stocks) employing such four-in-one type power semiconductor module as disclosed in Patent Document 1. FIG. 4 shows a whole assembled inverter unit; FIG. 5 is an external perspective view of an IGBT module that is a power semiconductor module for one phase (U-phase) of the inverter unit; FIG. 6 is a plan view of the IGBT module of FIG. 5; and FIG. 7 is a circuit diagram of the module.
Referring to FIG. 4, the reference numeral 1 shows an IGBT module, the reference numeral 2 shows a heat sink (cooling fins), the reference numeral 3 shows a laminated bus bar composing a circuit board of the unit, and reference numeral 4 shows a smoothing capacitor of a DC power supply connecting to the input side of the IGBT module 1. The IGBT module 1 contains four switching elements T1 through T4 indicated in the circuit diagram of FIG. 7 housed in the molded resin casing 5 of a box type with a rectangular planar configuration (see FIG. 5 and FIG. 6). Main circuit terminals 7 drawn out from the switching elements T1 through T4 are arranged in a row on protruding terminal block 5a formed on the central region of a top surface of the molded resin casing 5. Control terminals 8 drawn out from the switching elements are arranged in a row on a terminal block 5b formed at a side edge (in a shorter side) of the molded resin casing 5. The reference numeral 5c shows a screw holder for a-mounting screw formed at four corners of the molded resin casing 5; the reference numeral 5c-1 shows a metal sleeve inserted in a screw hole of the screw holder 5c; and the reference numeral 6 shows a metal base plate disposed on the bottom surface of the molded resin casing 5. The IGBT module 1 is fixed to the heat sink 2 (see FIG. 4) with mounting screws 9 inserted through the screw holder 5c. 
FIG. 7 illustrates an internal circuit diagram of the IGBT module. Referring to FIG. 7, each set of the four switching elements T1 through T4 has an IGBT and a FWD in anti-parallel connection. A pair of the switching elements T1 and T2 is connected in series between a positive terminal P and a negative terminal N of a DC power supply and forms an upper arm and a lower arm of a one phase of an inverter unit. Another pair of the other switching elements T3 and T4 is connected in anti-series connection and connecting to an AC output terminal U and the neutral terminal M of the DC power supply forming a bidirectional switching circuit. Operation of the three-level IGBT module is described in detail in Patent Document 1 so it is not described here.
FIG. 7 indicates symbols of terminals ‘T1 G’, ‘T1/T4 E’, ‘T2 G’, ‘T2 E’, ‘T3 G’, ‘T3 E’, ‘T4 G’, ‘T4 E’, and ‘C’, which are control terminals (sensing terminals) drawn out corresponding to the sets of switching elements T1 through T4. The symbols of terminals are indicated corresponding to FIG. 7 at the main circuit terminals 7 and the control terminals 8 in FIG. 6. In actual products, the symbols of terminals are indicated by stamping at the sides of corresponding terminals 7 and 8 on the top surface of the molded resin casing 5.
Meanwhile, there are guidelines for electric insulation of the power IGBT module: a domestic regulation according to JEM standards (Standards of The Japan Electrical Manufacturers' Association) and an international regulation according to IEC standards (Standards of the International Electro technical Commission). These standards specify a creepage distance and a clearance for electric insulation corresponding to the operational environment and the working voltage of the semiconductor module. For devices including an IGBT module for application to rolling stocks in particular, the annex of the International Standard IEC 60077-1 (Railway applications—Electric equipment for rolling stock) specifies insulation distances (the creepage distance and the clearance) corresponding to an overhead line voltage and degree of contamination of the operation environment.
If no restriction is imposed on the external size of the module, the required insulation distance can be ensured without problem between a terminal of the main circuit terminals 7 and the control terminals 8 disposed on the top surface of the molded resin casing 5 in FIGS. 5 and 6 and the metal base plate 6, and between the terminal and the mounting screw 9 (which is at the ground frame potential); and also between any two of the terminals, by simply enlarging the size of the module itself.
When reduction of the size or thickness of the module is desired, however, it is difficult to ensure a required creepage distance and a clearance with a flat insulation plane of the casing top surface alone because of the limitation of the external size. The control terminals 8 in particular, which are drawn out from the switching elements T1 through T4 and arranged in a row on the terminal block 5b of the molded resin casing 5, can hardly ensure the required insulation distance with a configuration as it is, because a gap between the terminals is narrow, and the terminals are located in the proximity of the mounting screws 9 at the ground frame potential in the both sides of the terminal block 5b. 
In order to cope with this insulation problem, Patent Document 2 discloses a module packaging structure in which electrically insulating separation walls (barriers) are formed standing at the periphery of a molded resin casing surrounding main circuit terminals and control terminals disposed on the casing top surface, thereby ensuring the required creepage distance and the clearance from the terminals to a metal base plate and mounting screws disposed on the bottom region of the casing.
In view of the background, the inventors of the present invention fabricated a package structure and studied the insulation performance thereof. The fabricated package structure is basically similar to that of the module shown in FIGS. 5 and 6, and has an enhanced insulation distance between the control terminals 8 and the ground frame potential components of metal base plate 6 and mounting screws 9, and between the control terminals. This enhancement of insulation distance was intended to be achieved, as shown in FIG. 8, by forming peripheral walls 5b-1 standing surrounding the control terminals 8 corresponding to the switching elements T1 through T4 around the terminal block 5b at the side edge of the casing having the control terminals 8.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2008-193779
[Patent. Document 2] Japanese Patent No. 3705755
The IGBT module of FIG. 8 as described above, though exhibiting an enhanced insulation performance for the control terminals 8 as compared with the conventional structure of FIG. 5 and FIG. 6, has been found to involve the following new problems with regard to application to an auxiliary power supply for rolling stocks of DC 750 V input.    (1) The insulating separation walls 5b-1 encircling the control terminals 8 and surrounding the entire periphery of the terminal block 5b provided at the side edge of the molded resin casing 5 decreases a space for indicating the symbols of the control terminals to be stamped on the terminal block surface which is shrunk due to the thickness of the newly formed surrounding walls of the insulating separation walls 5b-1. As a consequence, the letters for indicating the symbols of terminals must be made smaller and in addition, the surrounding walls obstruct visual recognition of the terminal symbols. Thus, any erroneous wiring may occur in the wiring work of the inverter unit. If the terminal block 5b is expanded preferring obtaining sufficient space for indicating the terminal symbols on the other hand, the external dimensions of the molded resin casing 5 increases, suppressing an area for packaging the IGBT module.    (2) The standards of the IEC 60077-1 (The Standards for rolling stocks) specifies the minimum width (x) of a groove formed on a surface of a molded resin casing to be 2.5 mm. A gap or groove less than this width is not regarded as a creepage distance. Moreover, a creepage distance between an electrically conductive part and a ground potential metal must be at least 16 mm in a device of an insulation voltage of 1,000 V class. As a consequence, when a clearance of at least 2.5 mm is contemplated, which is the minimum width (x) according to the standards above-mentioned between every control terminal 8 disposed on the terminal block 5b and the surrounding insulating separation walls 5b-1, the external size of the molded resin casing 5 must increase and enlarges the module size, which is another problem.